There are many uses for an apparatus which can produce an inverted logarithmic S curve of gain versus potentiometer rotation while using a linear resistance taper potentiometer. Prior art did not produce this curve and used only passive components such as T-pads with special taper characteristics. Disadvantages of the prior art include cost of special taper components, amplitude loss rather than gain, and strict termination impedance requirements. Advantages of the invention over the prior art include active gain, selectable impedance for input and output without affecting performance and most importantly, the use of a standard linear taper potentiometer.